1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device having at least two insulated gate field effect transistor portions for allowing a main current to flow between opposing main surfaces of a semiconductor substrate.
2. Description of the Background Art
In the field of a high withstand-voltage semiconductor device for controlling a voltage exceeding several hundred V, a current handled thereby is also high and hence element characteristics achieving suppressed heat emission, that is, loss, are required. In addition, as a method for driving a gate for controlling such a voltage and current, a voltage driven element having a small drive circuit and being low in loss therein is desirable.
For the reasons as described above, currently in this field, an insulated gate bipolar transistor, that is, an IGBT, has become mainstream as an element that can be driven with a voltage and be low in loss. A structure of this IGBT is a structure that can be regarded as keeping a withstand voltage by lowering impurity concentration in a drain of a MOS (Metal Oxide Semiconductor) transistor and having a drain side as a diode in order to lower drain resistance.
In such an IGBT, since the diode performs a bipolar operation, a source of the MOS transistor of the IGBT is herein called an emitter and the drain side is called a collector side.
In the IGBT representing a voltage driven element, generally, a voltage of several hundred V is applied across the collector and the emitter and the voltage is controlled by a gate voltage from ±several V to several ten V. In many cases, an IGBT is used as a switching element in an inverter. While the IGBT is in an ON state, a high current flows between the collector and the emitter and a voltage across the collector and the emitter becomes lower. While the IGBT is in an OFF state, little current flows between the collector and the emitter and a voltage across the collector and the emitter becomes higher.
Normally, as an IGBT operates as described above, loss in the IGBT can be categorized into ON-state power dissipation which is a product of a current and a voltage during the ON state and switching loss at the time of transition at which switching between the ON state and the OFF state is made. Since a product of leakage current and voltage during the OFF state is very small, it is ignorable.
On the other hand, even in an abnormal state such as short-circuiting of a load, it is important also to prevent an element from breaking down. In such a case, a gate is turned on while a power supply voltage of several hundred V is applied across the collector and the emitter and a high current flows.
In an IGBT having such a structure that a MOS transistor and a diode are connected in series, a maximum current is restricted by a saturation current of the MOS transistor. Therefore, even at the time of short-circuiting as above, a current is restricted and breakdown of an element due to heat emission can be prevented for a certain period of time.
In a recent IGBT, in order to further decrease loss, a trench gate type IGBT adopting a trench gate made by forming a trench in a surface of an element and embedding a gate electrode therein has become mainstream (see, for example, Japanese Patent Laying-Open Nos. 9-331063, 8-167711, 11-330466, 2010-10556, 2002-16252, and 2001-244325). Since a trench gate type IGBT is an element in which a MOS transistor portion has been reduced in size, its gate capacitance has been increased. In addition, a saturation current becomes very high at the time of short-circuiting, and therefore heat emission is great and the IGBT tends to break down in a short period of time.
Moreover, in recent years, as described, for example, in a document “Proceedings of 1998 International Symposium on Power Semiconductor Devices & ICs, p. 89.” such a phenomenon that oscillation occurs in a gate voltage, a gate current, a collector/emitter voltage, and a collector current at the time of short-circuiting due to a feedback capacitance of an IGBT which results in a malfunction has been known. Such an oscillation phenomenon due to a feedback capacitance has become a serious problem in an element having a large gate capacitance such as a trench gate type IGBT.
In order to address such problems, a structure in which a gate capacitance is suppressed by employing a dummy trench which is a trench not electrically connected to a gate electrode has been proposed. Further, WO02/058160 has proposed a structure allowing suppression of oscillation at the time of short-circuiting.
In the conventional examples above, as a ratio of dummy trenches (hereinafter referred to as a stabilizing plate or a trench for stabilizing plate) is increased in a high withstand-voltage semiconductor device such as a trench gate type IGBT in order to suppress oscillation at the time of short-circuiting, an ON voltage (Vce(sat)) and ON-state power dissipation increase and hence the number of dummy trenches cannot sufficiently be increased.